1. Field of the Invention
This invention relates generally to systems for drilling boreholes for the production of hydrocarbons and more particularly to a drilling system having an acoustic measurement-while-drilling ("MWD") system as part of a bottomhole assembly for measuring acoustic velocities of subsurface formations during drilling of the wellbores and determining the location of formation bed boundaries around the bottomhole assembly. Specifically, this invention relates to the imaging of bed boundaries using semblance techniques in an MWD system. For the purposes of this invention, the term "bed boundary" is used to denote a geologic bed boundary, interface between layers having an acoustic impedance contrast, or a subsurface reflection point.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes or wellbores are drilled through hydrocarbon-bearing subsurface formations. A large number of the current drilling activity involves drilling "horizontal" boreholes. Advances in the MWD measurements and drill bit steering systems placed in the drill string enable drilling of the horizontal boreholes with enhanced efficiency and greater success. Recently, horizontal boreholes, extending several thousand meters ("extended reach" boreholes), have been drilled to access hydrocarbon reserves at reservoir flanks and to develop satellite fields from existing offshore platforms. Even more recently, attempts have been made to drill boreholes corresponding to three-dimensional borehole profiles. Such borehole profiles often include several builds and turns along the drill path. Such three dimensional borehole profiles allow hydrocarbon recovery from multiple formations and allow optimal placement of wellbores in geologically intricate formations.
Hydrocarbon recovery can be maximized by drilling the horizontal and complex wellbores along optimal locations within the hydrocarbon-producing formations (payzones). Crucial to the success of these wellbores is (1) to establish reliable stratigraphic position control while landing the wellbore into the target formation and (2) to properly navigate the drill bit through the formation during drilling. In order to achieve such wellbore profiles, it is important to determine the true location of the drill bit relative to the formation bed boundaries and boundaries between the various fluids, such as the oil, gas and water. Lack of such information can lead to severe "dogleg" paths along the borehole resulting from hole or drill path corrections to find or to reenter the payzones. Such wellbore profiles usually limit the horizontal reach and the final wellbore length exposed to the reservoir. Optimization of the borehole location within the formation can also have a substantial impact on maximizing production rates and minimizing gas and water coning problems. Steering efficiency and geological positioning are considered in the industry among the greatest limitations of the current drilling systems for drilling horizontal and complex wellbores. Availability of relatively precise three-dimensional subsurface seismic maps, location of the drilling assembly relative to the bed boundaries of the formation around the drilling assembly can greatly enhance the chances of drilling boreholes for maximum recovery. Prior art downhole lack in providing such information during drilling of the boreholes.
Modern directional drilling systems usually employ a drill string having a drill bit at the bottom that is rotated by a drill motor (commonly referred to as the "mud motor"). A plurality of sensors and MWD devices are placed in close proximity to the drill bit to measure certain drilling, borehole and formation evaluation parameters. Such parameters are then utilized to navigate the drill bit along a desired drill path. Typically, sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a formation resistivity measuring device are employed to determine the drill string and borehole-related parameters. The resistivity measurements are used to determine the presence of hydrocarbons against water around and/or a short distance in front of the drill bit. Resistivity measurements are most commonly utilized to navigate or "geosteer" the drill bit. However, the depth of investigation of the resistivity devices usually extends to 2-3 meters. Resistivity measurements do not provide bed boundary information relative to the downhole subassembly. Furthermore, error margin of the depth-measuring devices, usually deployed on the surface, is frequently greater than the depth of investigation of the resistivity devices. Thus, it is desirable to have a downhole system which can relatively accurately map the bed boundaries around the downhole subassembly so that the drill string may be steered to obtain optimal borehole trajectories.
Thus, the relative position uncertainty of the wellbore being drilled and the critical near-wellbore bed boundary or contact is defined by the accuracy of the MWD directional survey tools and the formation dip uncertainty. MWD tools are deployed to measure the earth's gravity and magnetic field to determine the inclination and azimuth. Knowledge of the course and position of the wellbore depends entirely on these two angles. Under normal operating conditions, the inclination measurement accuracy is approximately plus or minus 0.2.degree.. Such an error translates into a target location uncertainty of about 3.0 meters per 1000 meters along the borehole. Additionally, dip rate variations of several degrees are common. The optimal placement of the borehole is thus very difficult to obtain based on the currently available MWD measurements, particularly in thin payzones, dipping formation and complex wellbore designs.
Recently, PCT application No. PCT/NO/00183 filed by Statoil Corp. disclosed the use of acoustic sensors having a relatively short spacing between the receivers and the transmitter to determine the formation bed boundaries around the downhole subassembly. An essential element in determining the bed boundaries is the determination of the travel time of the reflection acoustic signals from the bed boundaries or other interface anomalies. This application proposes utilizing estimates of the acoustic velocities obtained from prior seismic data or offset wells. Such acoustic velocities are not very precise because they are estimates of actual formation acoustic velocities. Also, since the depth measurements can be off by several meters from the true depth of the downhole subassembly, it is highly desirable to utilize actual acoustic formation velocities determined downhole during the drilling operations to determine the location of bed boundaries relative to the drill bit location in the wellbore.
Additionally, for acoustic or sonic sensor measurements, the most significant noise source is due to acoustic signals traveling from the source to the receivers via the metallic tool housing (commonly referred to as the "body waves") and the mud column surrounding the downhole subassembly (commonly referred to as the "tube waves"). The Statoil application discloses acoustic sensor designs to achieve a certain amount of directivity of signals. It also discloses a transmitter coupling scheme and signal processing method for reducing the effects of the tube wave and the body waves. Such methods, however, alone do not provide sufficient reduction of the tube and body wave effects, especially due to strong direct coupling of the acoustic signals between the transmitters and their associated receivers.
The present invention addresses the above-noted needs and provides a system for drilling boreholes wherein the bottomhole subassembly includes an acoustic MWD system having one acoustic sensor arrangement that is utilized to determine the acoustic velocities of the borehole formations during drilling and another acoustic sensor arrangement for determining bed boundary information based on the formation acoustic velocities measured downhole. Novel acoustic sensor arrangements are disclosed for relatively precisely determining the bed boundary information. A semblance based technique processes the measured reflections from the bed boundaries and determines the position and orientation of the bed boundaries with respect to the borehole tool. Acoustic isolators between the transmitters and their associated receivers are provided to reduce the body wave and tube wave effects. Any number of additional MWD devices or sensors may be included in the bottomhole assembly to obtain additional information about the borehole and the surrounding formations. A steering device or system is included in the bottomhole assembly which can be operated downhole and/or from the surface to steer the drill bit to drill the wellbore along the desired path.
The system of the present invention correlates measurements from the various MWD devices and sensors to provide parameters of interest relating to the drilling operations and formation evaluation. The bed boundary information may be utilized to map the borehole profile, to update or modify seismic data stored in the downhole subassembly and to steer the drill bit so as to obtain the desired borehole profile. The bed boundary and other information computed downhole may be stored downhole for later retrieval and use. Additionally, selected parameters of interest and other information are transmitted to the surface during the drilling operations to aid the driller in controlling the drilling operations including accurately geosteering the drill string.